Method for inhibiting scale formation using calcium hypochlorite compositions

ABSTRACT

A solid calcium hypochlorite composition contains hydrated calcium hypochlorite and an alkali metal salt of phosphonobutane polycarboxylic acid having an available chlorine concentration of at least 50 percent by weight and a water content of at least 4 percent by weight. It has been found that the use of the novel compositions of the present invention can significantly reduce scale formation in dispensers for calcium hypochlorite particularly where water having high total alkalinity is used. The prevention or inhibition of scale formation is accomplished without harmfully affecting other properties of the pool water such as the pH.

This invention relates to calcium hypochlorite compositions. Moreparticularly, this invention relates to improved calcium hypochloritecompositions for disinfecting and sanitizing water supplies.

Calcium hypochlorite is widely used as a disinfectant and sanitizingagent for supplying available chlorine in the treatment of watersupplies such as swimming pool water. To sanitize swimming pool water,available chlorine concentrations ranging from less than 1 part permillion to a few parts per million are continually maintained. Inconventional methods of application, granular calcium hypochlorite isperiodically added directly to the water in the pool in quantitiessufficient to maintain the available chlorine at the desired levels. Itis preferred, however, to provide continuous contact between the poolwater and the solid calcium hypochlorite. Placing tablets of calciumhypochlorite in the skimmer or in dissolving baskets around the pool isone method employed. Another method used is to add solid calciumhypochlorite to a dispensing device in which the calcium hypochlorite iscontacted with the water to be treated so that the dissolving of thesolid is controlled to form a solution of the desired available chlorineconcentration. This concentrated solution is then added to the totalbody of pool water to provide the desired available chlorineconcentration.

The operation of these dispensers in treating pool water issubstantially trouble-free where the total alkalinity of the water isless than about 100 ppm (expressed as calcium carbonate).

Where the water has a total alkalinity in excess of about 100 ppm (asCaCO₃) and particularly where the pH is high, for example, in excess of7.8, there is a tendency for the formation and build-up of scale whenusing dispensers for calcium hypochlorite. Scale build-up is deleteriousin that it can block or plug up drains and outlets in the dispenser sothat solution flow is deterred or stopped. This results in inconsistentcontrol of the available chlorine concentration in the water body.Removal of this scale requires, for example, disassembly of thedispensing device and manual cleaning with strong mineral acid. Thisprocess is both time consuming and potentially hazardous.

Additives may be introduced to limit the formation of scale, forexample, additives which reduce the PH can be added to the pool water orto the dispenser. The use of chemical additives of the scale inhibitoror dispersant tYpes have been shown to control scale in numerousindustrial applications. Direct addition of these additives to the poolwater, while possible, is not preferred, for example, as a much higherlevel of additive is required and maintenance of the proper additivelevel by direct addition to the dispenser is much more difficult toachieve.

Ideally it is desired to have a calcium hypochlorite composition whichincludes the additive to prevent or inhibit scale formation. Inpractice, however, it is difficult to find additives which can bedirectly admixed with calcium hypochlorite. Calcium hypochlorite is ahighly active inorganic oxidizing agent which can react readily withoxidizable substances, such as organic substances, in anoxidation-reduction reaction. The addition of chemical compounds,particularly organic compounds, to solid calcium hypochlorite istherefore generally not practiced as the high reactivity of theresultant mixture can result in the release of toxic gases, fire orexplosion.

H. Geffers et al in German Patent 2,141,984, issued Aug. 12, 1976teaches that the use of phosphonopolycarboxylic acids or their salts inactive chlorine containing solutions provides good water-softeningaction. Phosphonosuccinic acid or its sodium salt was added to alkalinechlorine bleach solutions in amounts of at least 5% by weight. Gefferset al also prepared a mixture of 90% of the Na salt of2-phosphonobutane-1,2,4-tricarboxylic acid and 10% sodiumdichloroisocyanurate which was added to water used in bottle washingmachines.

The high degree of reactivity of solid calcium hypochlorite, however, isnot encountered in aqueous solutions of hypochlorite compounds becauseof the much lower available chlorine concentrations and the moderatingeffect of water.

Further, mixtures containing high concentrations ofphosphonopolycarboxylic acids or their salts are unsuitable for use insanitizing and disinfecting water supplies as large amounts of thecompositions are required which result in significant cost increases forwater treatment.

Surprisingly it has now been found that calcium hypochlorite can bedirectly admixed with an alkali metal salt of a phosphonobutanepolycarboxylic acid to produce dry solid stable compositions having highavailable chlorine concentrations for sanitizing and disinfecting waterbodies while preventing the formation of scale in apparatus used inwater treatment.

One component of the novel compositions of the present invention iscalcium hypochlorite. Calcium hypochlorite compounds suitable for useinclude those having an available chlorine concentration of at least 50%by weight and are "hydrated", having a water content of at least 4percent by weight. Preferably the calcium hypochlorite compounds have anavailable chlorine concentration of at least 65% and a water content inthe range of from about 6 to about 18 percent, and more preferably theavailable chlorine concentration is at least 70% by weight and the watercontent is from about 6 to about 12 percent by weight.

Admixed with the calcium hypochlorite compound is an alkali metal saltof a phosphonobutane polycarboxylic acid represented by the formula:##STR1## in which R represents H or CHR'--CHR"--CO--OH, R' represents Hor CO--OH, and

R" represents H or a lower alkyl group.

The alkali metal salts of these phosphonobutane polycarboxylic acidswhich can be employed include sodium, potassium, lithium, and mixturesthereof, among others. Preferred as alkali metals for reasons ofavailability and economy are sodium and potassium, with sodium beingparticularly preferred.

The phosphonobutane polycarboxylic acids represented by formula (I)include phosphonobutane dicarboxylic acid, phosphonobutane tricarboxylicacid, and phosphonobutane tetracarboxylic acid which may be substitutedby lower alkyl groups (R") having up to about 4 carbon atoms such asmethyl,ethyl, propyl, and butyl. Of these polycarboxylic acid compounds,preferred are those in which R represents CHR'--CHR"--CO--OH and R'represents H or CO--OH, and more preferred is phosphonobutanetricarboxylic acid in which R' represents H and R" represents H.

The calcium hypochlorite compositions of the present invention includeamounts of the alkali metal salt of the phosphonobutane polycarboxYlicacid which are sufficient to inhibit or prevent scale formation, forexample, on apparatus used in the sanitation of swimming pool waterwhile providing high concentrations of available chlorine forsanitation. The compositions employ concentrations of the alkali metalsalt of the phosphonobutane polycarboxylic acid of at least 0.005percent by weight, for example, those in the range of from about 0.005to about 5 percent by weight.

Preferred concentrations for both operative and economic reasons arethose in the range of from about 0.01 to about 3, and more preferablyfrom about 0.1 to about 1.5 percent by weight.

The calcium hypochlorite compositions are prepared by admixing thecalcium hypochlorite with the alkali metal salt of the phosphonobutanepolycarboxylic acid in any suitable manner.

In one embodiment calcium hypochlorite is admixed with the alkali metalsalt of the phosphonobutane polycarboxylic acid to produce a dry blendedproduct suitable for addition to water bodies such as swimming pools tosanitize the pool water. Any suitable means of mixing or blending thecomponents of the compositions may be used including, for example,rotary drums or cylinders, inclined discs or pans, ribbon or paddle typeblenders, centrifuges, planetary mixers, spiral elevators, fluid bedmixers, and the like.

Further, the novel compositions of the present invention may beagglomerated, for example by spray drying or spray graining or by otherknown methods such as those described in the Kirk-Othmer Encyclopedia ofChemical Technology, 3rd edition, volume 21, pages 82-89 (New York, JohnWiley, 1983).

Preferred embodiments are compressed forms of the novel calciumhypochlorite compositions which can be used in a dispensing device whichcontrols the dissolution of the composition, for example, by limiting insome manner contact of the water with the calcium hypochloritecomposition. Suitable compressed forms include tablets, briquets, sticksor cylinders, pellets and the like.

These compressed forms are produced by known methods such as tablettingor briquetting. Of these compressed forms of the novel compositions ofthe present invention tablets or briquets are favored.

In an alternate embodiment, compressed forms of calcium hypochlorite maybe contacted with the alkali metal salt of the phosphonobutanepolycarboxylic acid to form a coating on the surface of the calciumhypochlorite.

Compressed forms of the novel calcium hypochlorite compositions can beused in any dispensing device which controls the dissolution of thesolid composition. Typical examples include those of U.S. Pat. Nos.2,700,651; 2,738,323; 3,416,897; 3,598,536; 3,607,103; 3,615,244;3,638,833; 3,727,632; 3,802,845; 3,860,394; 3,864,090; 3,870,471;3,912,627; 4,208,376; 4,374,563; 4,546,503; 4,643,881; D288,226; andD297,857 among others.

It has been found that the use of the novel compositions of the presentinvention can significantly reduce scale formation in dispensers forcalcium hypochlorite particularly where water having high totalalkalinity is used. The prevention or inhibition of scale formation isaccomplished without harmfully affecting other properties of the poolwater such as the pH, and thus does not promote the corrosion of metalssuch as Fe, Cu or Al used in components of the pool or the recirculationsystem. During storage neither component of the product undergoessignificant decomposition which would result in the substantial loss ofavailable chlorine or the breakdown of the phosphonobutanepolycarboxylic acid.

The use of the novel composition of the present invention is compatiblein aqueous solutions containing other additives to swimming pool water,for example, algaecides, stabilizing agents such as cyanuric acid, pHadjustment agents, for example sodium bisulfate and sodium carbonate,and bromine compounds such as bromochlorodimethyl hydantoin. Inaddition, there is no harmful build-up of the phosphonobutanepolycarboxylic acid compound in the pool water.

The following examples are presented to further illustrate the inventionwithout any intention of being limited thereby. All parts andpercentages are by weight unless otherwise specified.

EXAMPLE 1

Commercial grade hydrated calcium hypochlorite (19.8 g.) containing 65%of available chlorine and 6% of water, was mixed with 0.2 grams of thesodium salt of 2-phosphonobutane 1,2,4-tricarboxylic acid (Na PBTC) andthe granular mixture placed in a vial. The vial was sealed and stored inan oven at 45° C. for 30 days. At the end of this period, the sample wasseparated into components by sieving and the appropriate componentanalyzed. Available chlorine loss was determined by iodometric titrationand PBTC loss by high pressure ion chromatography (HPIC) with aconductivity detector. The results are given in Table I below:

Comparative Example A

The procedure of Example 1 was repeated exactly using the sodium salt ofhydroxyethylidene diphosphonic acid (HEDP), a commercially availablescale inhibitor, as the additive. The results are given in Table Ibelow.

                  TABLE I                                                         ______________________________________                                                                Available                                                                     Chlorine                                                                      Loss Relative                                                                             Additive                                  Example  Additive       to Control  Loss %                                    ______________________________________                                        Control  None           1.00        NA                                        1        Na 2-phosphonobutane                                                                         0.98         5.92                                              1,2,4-tricarboxylate                                                 Comp. A. Na Hydroxyethylidene                                                                         0.96        82.27                                              Diphosphonate                                                        ______________________________________                                    

As shown in Table I, little decomposition of the additive resulted inthe composition of the invention using the sodium salt of2-phosphonobutane 1,2,4-tricarboxylic acid. In contrast, decompositionof the sodium salt of hydroxyethylidene diphosphonic acid took place tothe extent that this material would be unsuitable as an additive.

EXAMPLE 2

The hydrated calcium hypochlorite product of Example 1 (99 lbs, 45 kgs)was blended with 1 lb (0.45 kg) of the sodium salt of 2-phosphonobutane1,2,4-tricarboxylic acid to produce a homogeneous mixture. The mixturewas then tabletted on a Stokes press to form 7 gram tablets having adiameter of 0.75 inch (1.9 centimeters). The tablets were loaded into adispenser of the type taught in U.S. Ser. No. 07/238,446, filed Aug. 31,1988 by C. M. Zetena and R. P. Alexander. The dispenser included threechambers: a chemical chamber, a dissolving chamber, and a dischargechamber. The chemical chamber extended down into the dissolving chamberwhich overlay and was in flow communication with the discharge chamber.The chemical chamber contained a perforated grid onto which the tabletswere placed The level of water which flowed into the dissolving chamberand which contacted the tablets on the perforated grid was controlled bya vertically adjustable level controller which also controlled therelease of treated water from the dissolving chamber into the dischargechamber. Swimming pool water having a pH in the range of 7.8-8.2 and ata temperature of 85±2° F. (30±° C.) was continuously introduced into thedissolving chamber.

The swimming pool water had a total alkalinity of 120-160 ppm CaCO₃ anda calcium hardness of 300-500 ppm as CaCO₃. The vertical adjustablecontrol means included a siphon tube for discharging treated water fromthe dissolving chamber to the discharge chamber. The vertical adjustablecontrol means was set to provide a dissolving rate for the tablets of 10lbs/day (4.5 kg/day). After all of the tablets had been dissolved, thesiphon tube was removed and placed in 800 mls of dilute hydrochloricacid (1:10) to dissolve any scale present. De-ionized water was added tothe hydrochloric acid solution to provide 1 liter of solution. Thesolution was analyzed for calcium by Atomic Absorption. The results aregiven in Table II below.

                  TABLE II                                                        ______________________________________                                                                              % Scale                                           Wt % of  Wt % of    Scale   Reduc-                                  Example No.                                                                             Ca(OCl).sub.2                                                                          Na PBTC    (g. CaCO.sub.3)                                                                       tion                                    ______________________________________                                        Control   100      --         11.0    NA                                      2          99      1          0.37    97                                      ______________________________________                                    

EXAMPLE 3-4

The procedure of Example 2 was repeated with 50 lbs (23 kgs) of tabletscontaining 0.7% and 0.3 percent of Na PBTC respectively. The dissolvingrate of the tablets was set at 12 lbs (5.5 kgs) per day. The results aregiven in Table III.

                  TABLE III                                                       ______________________________________                                                                              % Scale                                           Wt % of  Wt % of    Scale   Reduc-                                  Example No.                                                                             Ca(OCl).sub.2                                                                          Na PBTC    (g. CaCO.sub.3)                                                                       tion                                    ______________________________________                                        Control    100     --         4.6     NA                                      3         99.3     0.7         0.68   85                                      4         99.7     0.3        1.5     67                                      ______________________________________                                    

EXAMPLE 5

The procedure of Example 2 was repeated using 100 lbs of tabletscontaining 0.1% by weight of Na PBTC. The tablets were dissolved at arate of 15 lb. (6.8 kg) per day. The results are given in Table IVbelow.

                  TABLE IV                                                        ______________________________________                                                                              % Scale                                           Wt % of  Wt % of    Scale   Reduc-                                  Example No.                                                                             Ca(OCl).sub.2                                                                          Na PBTC    (g. CaCO.sub.3)                                                                       tion                                    ______________________________________                                        Control    100     --         6.8     NA                                      5         99.9     0.1        6.4     5.3                                     ______________________________________                                    

EXAMPLE 6

Tablets containing 99.25% hydrated calcium hypochlorite and 0.75% NaPBTCwere produced. The tablets were used in a dispenser of the type employedin Example 2. The dispenser was used to supply available chlorine towater in a 72,000 gallon swimming pool. The dispenser was operated forthree weeks and in that time period almost no scaling was observed, no"down-time" was experienced, and no cleaning was required at the end ofthe three week period.

Comparative Example B

The procedure of Example 6 was employed using tablets of calciumhypochlorite without the scale inhibiting additive in the dispenser.During operation, severe scaling of various parts of the dispenserresulted, requiring disassembly and cleaning of the dispenser on aweekly basis.

While the invention has been described above with reference to specificembodiments thereof, it is apparent that many changes, modifications andvariations can be made without departing from the inventive conceptdisclosed herein. Accordingly, it is intended to embrace all suchchanges, modifications and variations that fall within the spirit andbroad scope of the appended claims. All patent applications, patents andother publications cited herein are incorporated by reference in theirentirety.

What is claimed is:
 1. A method for inhibiting scale formation in adispensing means for adding calcium hypochlorite to water to form anaqueous solution of calcium hypochlorite which comprises:adding to thedispensing means a solid calcium hypochlorite composition comprisinghydrated calcium hypochlorite and an alkali metal salt ofphosphonobutane polycarboxylic acid having a concentration of at leastabout 0.005 percent by weight an available chlorine concentration of atleast 50 percent by weight and a water content of at least 4 percent bYweight, and contacting the solid calcium hypochlorite composition withwater to form the aqueous solution of calcium hypochlorite.
 2. Themethod of claim 1 in which the phosphonobutane polycarboxylic acid isrepresented by the formula: ##STR2## in which R represents H orCHR'--CHR"--CO--OH, R' represents H or CO--OH, andR" represents H or alower alkyl group.
 3. The method of claim 2 in which the alkali metal issodium, potassium, and mixtures thereof.
 4. The method of claim 3 inwhich the concentration of alkali metal salt of phosphonobutanepolycarboxylic acid is in the range of from about 0.1 to about 1.5percent by weight.
 5. The method of claim 4 in which the compressed formis a tablet or briquet.